look(): signal when the ref list is truncated

`look()` caps the snapshot at `MAX_REFS` (200) in `packages/cdp/src/snapshot.ts`. On long pages the agent can't tell there's more below, so it may assume it saw everything.

**Task:** when the snapshot is truncated, append a one-line hint to the output (e.g. `… N more elements below — scroll or filter`). Small, high-value, testable against the existing `compactAxTree` unit tests.